Materials in different stages of transformation from raw material to finished product often present a heterogeneous elemental composition. In particular, an object'surface may be functionalized using one or several layers of varying composition or physical properties. There is a growing need in industry, namely in the context of process development and process control, for at-site high-throughput methods that can reveal the distribution of one or several elements along one or more spatial dimensions.
Classical analytical techniques have largely focused on the determination of bulk compositions, and a few can provide spatially-resolved information. Typically, the material is dissolved and introduced as a solution in the analytical instrument, yielding only average elemental concentrations. Techniques based on an arc/spark do allow direct solid sampling (of electrically conducting materials) without a digestion step. However, they do not possess the capability to provide accurate spatially resolved analyses (Güther et al., Spectrochim Acta Part B, vol. 54, 1999, p. 381).
Other techniques, such as Auger or X-ray photoelectron spectrometry, allow the study of surface chemistry one the atomic scale, and can also provide depth-resolved analyses when removing successive layers of material through ion bombardment. In secondary ion mass spectrometry (SIMS), such a bombardment is inherent to the measurement process as the composition at different depths is inferred from the nature of bombardment-induced secondary ions. In order to avoid particle scattering in the gas phase, these and other techniques with similar attributes require working in ultra-high vacuum conditions. Glow-discharge methods coupled to optical emission spectrometry or mass spectrometry have also been used to probe coatings, over thicknesses ranging from 0.01 μm to 50 μm. Measurement time is typically 15 minutes and depth resolution is around 100 nm, while lateral resolution is poor.
The methods described above all involve some preparation of the sample, are time consuming, and require sophisticated and expensive instrumentation. Moreover, the sample shape and size is limited by the sample chamber configuration. Some also suffer from limited sensitivity or spatial resolution. For these reasons, they do not meet the industrial needs for at-site high-throughput compositional mapping of heterogeneous materials.
Laser radiation, on the other hand, possesses several attributes that make it particularly well suited for the task of analysing heterogeneous materials. In so-called laser ablation, a focused laser pulse provides locally a very large power density that transforms a small amount of solid material directly into a vapor plume suitable for further analysis. The possibility of concentrating laser radiation on a very small surface enables the sampling and analysis of heterogeneous materials with very good lateral resolution (down to a few micrometers). The separate analysis of successive laser ablation events at a same position on the solid material also enables a depth-resolved analysis, the depth reached by each laser shot ranging from tens of nanometers to tens of micrometers depending on the laser characteristics and material type.
Laser ablation (LA) in itself is not sufficient for compositional analysis. Two main schemes exist that can complement its role of direct solid sampling: i) the luminous plasma formed above the specimen surface concomitantly with the ablation event is analysed through optical emission spectrometry (OES), in a technique known as laser-induced plasma spectroscopy (LIPS), or (ii) the sampled matter is carried in a gas stream to an auxiliary inductively-coupled plasma (ICP) and detected through optical emission or mass spectrometry (in so-called hyphenated techniques known as LA-ICP-OES and LA-ICP-MS respectively). The first scheme (LIPS) is rapid, involves relatively simple instrumentation and analytical procedures, and is relatively inexpensive. Moreover, contrary to the second scheme which requires the sample to be enclosed in a laser ablation cell, LIPS can be applied to samples of any size or shape, and can function at a distance. Therefore, LIPS is the most amenable to at-site, in-situ, and high-throughput compositional mapping of heterogeneous materials.
Any compositional mapping of a solid material requires not only knowledge of the composition at a given analysis site but also an accurate knowledge of the site location in three-dimensional space. Knowledge of the laser impact site on the sample in the two dimensions transverse to the laser beam is easily gained by a precise and user-controlled steering of either the sample or the laser beam in these dimensions. Determining the position in the other direction, for example the distance between the bottom of the laser-produced crater and the sample surface beside the crater, is more difficult.
Following one or a sequence of laser ablation events, the sample can be taken to another instrument with which the crater depth will be determined. Such an instrument can be of a mechanical or optical type. In the first case, a fine point is moved across the surface of the sample, and the crater profile and depth is determined from the displacement of the point. For example, Kanický et al. (Fresenius J. Anal. Chem., vol. 366, 2000, p. 228) have used such a mechanical profilometer to assess the shape and depth of craters in the context of depth-profile analysis of tin-coated glass by LA-ICP-OES. In the second case, the instrument can be based on confocal microscopy, laser triangulation, or interferometry using a short coherence length light source (also called white light interferometry or optical coherence tomography). Wong et al. (SPIE, vol. 2390, 1995, p. 68) have used white light interferometry for the study of laser-ablation craters in bone in the context of laser treatment, not compositional analysis. Kay et al. (Int. J. Impact Engng., vol. 19, 1997, p. 739) have also used this technique for the characterization of impact (not laser-produced) craters. Borisov et al. (Spectrochim. Acta Part B, vol. 55, 2000, p. 1693) used a white-light interferometric microscope to study the parameters of laser-produced craters in the context of LA-ICP-MS analysis of a glass sample.
In order to establish a detailed depth profile, one needs to perform several compositional measurements at different depths in the material. To avoid repeatedly carrying the sample to a separate instrument for the determination of depth, and the subsequent need for precise positioning of the sample in the laser ablation apparatus, one can resort to a preestablished calibration of the crater depth on the basis of the cumulative number of laser shots. In this way, the compositional analysis for a given laser shot is made to correspond to a given depth. In cases where the sample comprises a coating and a substrate, both having significantly different ablation rates (ablated depth per laser shot), different calibrations can be used for the coating and substrate, and an interpolation can be used for the interface region. This procedure assumes that the ablation rate is the same for the study sample and the calibration sample, which in particular requires sufficient stability of the laser pulse energy and beam radial profile. However, this approach is limited to relatively simple cases. It would not be applicable to samples for which the ablation rate varies in a continuous manner as a function of depth, or to complex multilayer samples.
An example of such a problematic case is the compositional mapping of pharmaceutical tablets by LIPS. The core of pharmaceutical tablets consists of a compacted blend of different components (active agent, lubricant, inactive excipient, etc.) originally in powder form, and may be coated with a film (typically containing titanium dioxide and other ingredients). U.S. Pat. No. 5,781,289 Jul. 14, 1998 by Sabsabi and Bussiere describes the use of LIPS for the analysis of preselected components in homogeneous pharmaceutical compositions, for example for the quantification of the average active agent concentration in tablets. Although such a spatially-averaged analysis by LIPS can find several uses in pharmaceutical process development and control, a mapping capability would prove useful for another set of problems: (i) assessment of powder blend uniformity by the mapping in tablets of the drug, lubricant or other components, or (ii) evaluation of coating homogeneity and thickness across the surface of the tablet. In the latter case, a depth-profiling capability is required. However, because of the particular laser-matter interaction that occurs in tablets and of the granular nature of tablets, the corresponding ablation rate is usually very large compared to that in a metal for instance. Whereas on a metal tens of nanometers are ablated per laser shot, 10-15 μm can be ablated per pulse in a tablet coating and up to 50-100 μm per shot in the core of tablets. As a result, the aspect ratio (depth-to-diameter ratio) of the laser-produced crater can grow very large, thus significantly modifying the ablation rate at each successive shot (because of a decreasing laser energy density on the crater surface due to an increasing exposed surface, or because of increasing confinement of the ablated matter and of plasma in the crater). A depth calibration in this case would not be possible. The same would be true if instead of using LIPS, the analysis proceeded through the transfer of the ablated matter to an auxiliary discharge and detection system (as in LA-ICP-OES or LA-ICP-MS). The same would also be true of any other analysis based on direct solid sampling by laser ablation where the ablation rate varies continuously as a function of depth, or where the multilayer structure of the sample is so complex as to preclude any calibration.
Combining laser ablative sampling and optical sensing of the sampling position in a single integrated apparatus would provide a means of determining in real time the depth of laser-produced craters for each laser shot if desired, thus eliminating the need for depth calibration. U.S. Pat. No. 6,259,530 B1 Jul. 10, 2001 by Monsallut describes a method and device, based on optical heterodyne interferometry, for measuring the depth of craters obtained by the bombardment, with a beam of primary ions, of a sample placed in the analysis chamber of a physico-chemical analyzer, such as a SIMS instrument. This invention relates to depth-profile analysis by ion-based techniques in high-vacuum chambers. It does not feature an integrated optical system performing both functions of laser-ablative sampling and crater-depth evaluation. Moreover, this method requires the optical paths to follow an incident direction inclined in relation to the surface of the sample (thus freeing the space needed for the circulation of secondary ions extracted from the sample). Consequently, this configuration would not be adequate for the characterization of craters with large aspect ratio, since shading might occur.
Lausten and Balling (Appl. Phys. Lett., vol. 79, 2001, p. 884) describe a method for the real-time measurement of crater depth during ablation with ultrashort laser pulses, in the context of laser micromachining or laser surgery. The method is based on the time-gated imaging of the backscattered radiation from the ablation region. The crater shape is deduced from the time-of-flight of light to and from the object. For this reason, shorter pulses will provide better spatial resolution. However, even for a pulse as short as 100 fs (i.e. 10−13 s), the depth resolution is only about 15 μm, which would not be suitable for many depth-profile analysis applications. The method becomes wholly inapplicable with ns-duration (10−9 s) laser pulses widely used for LIPS, LA-ICP-OES or LA-ICP-MS. Therefore, there is a need to provide an optical tool for the non-contact, in-situ and real-time measurement of the depth of laser-produced craters, for each laser shot or at any shot number interval desired. The in situ and real time measurement of depth eliminates the need to periodically characterize the crater depth in another separately-located instrument, or to rely on a calibration of depth (based on cumulative shot number) for a given material, or finally to resort to an interpolation of such calibrations for describing the interface between two materials.